It is known that an underwater vessel (i.e., a submarine) generates sound, which is generally referred to as passive sound, as it travels through the water. The passive sound is generated by a variety of sources, including, but not limited to, sound generated by a submarine propulsion system, sound generated by a submarine propeller, and sound generated by a submarine electrical power generator. It is known that submarine designers attempt to reduce these and other passive sound sources in order to make a submarine difficult to detect by passive acoustic means, therefore remaining as covert as possible.
Some anti-submarine warfare (ASW) sonar systems attempt to detect the passive underwater sound generated by an enemy submarine. Some other ASW sonar systems attempt to both detect the passive sound and also to localize and/or track the enemy submarine. Localization is used to identify a position of the enemy submarine in azimuth, and/or in range, and/or in depth.
Passive ASW sonar systems attempt to detect, localize, and/or track the enemy submarine using the received passive sound only. The passive sonar system can remain covert and undetectable by the enemy submarine. Some known passive sonar systems use beamforming techniques to generate receive beams. The receive beams can be steered azimuthally to detect, localize, and/or track the enemy submarine in azimuth. The receive beam can also be steered to vertical angles.
Even at relatively short ranges, localization in depth and range is not generally possible when receiving only passive sound and depending upon a pointing direction of receive beams (directed up or down). This is because for any receive beam and associated vertical beam that points toward an enemy submarine, the enemy submarine can be positioned at an essentially infinite number of depths and ranges along the vertical beam steer angle.
At longer ranges, localization of the enemy submarine in range and depth is made even more difficult by a variety of factors, including but not limited to, a tendency of the passive sound generated by the enemy submarine to bend (i.e., refract), primarily in a vertical direction, as the sound propagates through the water. Therefore, the vertical angle at which the greatest amount of sound arrives at the sonar system, which is related to a particular receive vertical beam angle, does not necessarily point in the direction of the enemy submarine. Therefore, other techniques must be used to localize the target in range and in depth.
Narrowband, or matched field processing, is a known technique used to localize in range and in depth. However, narrowband processing requires a large sonar array, which is not practical for many applications. Narrowband processing also suffers from the affects of the above-described sound refraction.
Broadband autocorrelation processing is a known technique in which a signal received by a sonar element (i.e., sonar transducer), or a sonar array, is autocorrelated to identify a relative time delay between the passive sound arriving at the sonar element on a direct sound path and the passive sound arriving at the sonar element on a surface-reflected sound path. The relative time delay can be used to calculate range and depth. However, the performance of this technique can be greatly degraded at moderate to high sea states (i.e., when the sea surface has high waves), due to scattering of the sound reflected from the surface, which causes the autocorrelation to degrade.
Though an actual range and depth localization of a sound-generating target is desirable, it is also desirable just to be able to discriminate a subsurface target, e.g., a submarine, from a surface target, e.g., a ship.